(a) Field of the Invention
The present invention relates to a driving circuit for a display device.
(b) Description of Related Art
In recent years, flat panel displays such as organic light emitting diode (“OLED”) displays, plasma display panels (“PDPs”) and liquid crystal displays (“LCDs”) have been widely developed for use instead of heavy and large cathode ray tubes (“CRTs”).
The PDP devices display characters or images using plasma generated by a gas discharge. The OLED display devices display characters or images by applying an electric field to specific light emitting organic or high molecule materials. The LCD devices display images by applying an electric field to a liquid crystal layer disposed between two panels and regulating the strength of the electric field to adjust transmittance of light passing through the liquid crystal layer.
Among the flat panel displays, as examples, the LCD and the OLED devices each include a panel assembly provided with pixels including switching elements and display signal lines. The LCD and the OLED devices also include a gate driver, i.e., a shift register that provides a gate signal for gate lines of the display signal lines to turn the switching elements on and off.
The shift register is comprised of a plurality of stages. Each of the stages includes a plurality of transistors, which generate an output synchronized with a clock signal of a plurality of clock signals in response to outputs of a previous stage and a next stage.
The shift register is integrated in the panel assembly that is formed together with the switching elements. In this case, each of the transistors comprising the stage contains a semiconductor layer, which may be constructed of an amorphous silicon or poly-silicon material. For example, when the amorphous silicon is used for the semiconductor layer, an ohmic contact layer and a drain electrode or a source electrode are sequentially formed thereon, and a gate insulating layer and a gate electrode are sequentially formed thereunder. When a predetermined voltage is applied to the gate electrode, a formation in the semiconductor layer causes currents to flow from the drain electrode to the source electrode.
In some cases when the shift register operates for a long time, the conductivity of each of the transistors may deteriorate, and thereby the shift register does not operate well.
In detail, when the gate voltage is low, electron concentration decreases due to an increase of a dangling bond in the semiconductor layer, and when the gate voltage is high, the electron concentration decreases due to what is called a tunneling phenomenon in which the electrons move to the gate insulating layer below the semiconductor layer. This causes the voltage difference over the gate insulating layer to be increased and thus a threshold voltage to be increased. Accordingly, a decrease of a drain voltage, which is proportional to the square of the difference between a gate-source voltage and the threshold voltage, does not generate a required output and causes the shift register to not operate well.